1. Field of the Invention
The present invention relates to an image information compression and decompression device in which a high definition (HD) image signal is processed by dividing it into a signal (compatible component) suitable for reproduction of a standard definition (SD) image and a signal (complementary component)which, when processed together with the compatible component signal, can reproduce the HD image. In particular, the present invention relates to an improvement of an image information compression and decompression device which is suitable when used together with a sub-band division and a motion compensation prediction coding for interframe or interfield movement.
2. Description of the Prior Art
In view of the widespread use of current television systems, it has been decided that both a SD image television system, such as NTSC, and a television system for HD image be utilized. In order to achieve compatibility between the HD image and the SD image, an image signal coding having hierarchy has been investigated. For example, in Sawada et al., "CTV/HDTV Compatible Coding Scheme," D-335, 1992, the Spring Meeting of the Electronic Information Communication Society of Japan, a pyramid coding system is disclosed in which a portion of a bit stream obtained by coding an HD image information is derived to obtain an SD image information.
This coding system will be described with reference to FIG. 1. An input HDTV signal is filtered and sub-sampled by a filter/sub-sampling circuit (100) and then coded by a coder (102). As a result, a coded component corresponding to the SD signal is output from the coder (102). The SD corresponding component is transmitted or recorded on the one hand, and locally decoded by a decoder (104), data-interpolated by an interpolation circuit (106) and supplied to a subtracter (108) on the other hand. In the subtracter (108), a difference component between the HDTV signal and the locally decoded signal is produced. That is, a difference component is obtained, which, when combined with the SD corresponding component, provides the HD image. The difference component is coded by a coder (110).
A sub-band coding system, shown in FIG. 2, which comprises a sub-band dividing circuit (144) and a sub-band combining circuit (180) is an example of the coding system having hierarchy. In the sub-band dividing circuit (144), shown in FIG. 2, an HDTV signal is band-divided horizontally by sub-band analysis filters (120) and (122), and outputs of these filters are sub-sampled by sub-sampling circuits (124) and (126), respectively. A low frequency band output from the sub-sampling circuit (124) is further band-divided vertically by sub-band filters (128) and (130). Output signals of the sub-band filters (128) and (130) are sub-sampled by sub-sampling circuits (132) and (134), respectively. On the other hand, a high frequency band output from the sub-sampling circuit (126) is further band-divided vertically by sub-band filters (136) and (138). Output signals of the sub-band filters (136) and (138) are sub-sampled by sub-sampling circuits (140) and (142), respectively.
Thus, horizontally and vertically band-divided signals LL, LH, HL, and HH are obtained by the sub-band dividing circuit (144). These band signals are suitably encoded (not shown), and then transmitted or recorded. FIG. 3 shows the band division. Among others, the LL band signal corresponds to the SD corresponding component and the remaining LH, HL, and HH band signals correspond to the difference component.
Returning to FIG. 2, the band signals LL, LH, HL, and HH received or reproduced are suitably decoded (not shown), interpolated by interpolation circuits (150-156), and then filtered by sub-band synthesis filters (158-164), respectively. Outputs of the sub-band synthesis filters (158) and (160) are combined vertically by an adder (166) and outputs of the combining filters (162) and (164) are combined vertically by an adder (168). After the combined outputs of the adders (166) and (168) are interpolated by interpolation circuits (170) and (172) and filtered by sub-band synthesis filters (174) and (176), respectively, they are combined horizontally by an adder (178). In this manner, the HD image signal is reconstructed by vertical and horizontal band-combining operation performed by the sub-band combining circuit (180). The SD image is restored from the LL component.
The number of band divisions in the above-mentioned example is 4. When the number of band divisions is to be 5 or more, respective band signals or the LL signal is processed repeatedly in the manner shown in FIG. 2.
On the other hand, the so-called MPEG-1 and MPEG-2 are methods for compressing image information utilizing correlation between image frames and correlation between pixels in a frame, respectively. For example, Yonemitsu et al., "MPEG Standard Proposal, Video Part (ISO 11172 VIDEO)," The Journal of Image Electronics Society, 20.4, pp. 306-316, August 1991, discloses MPEG-1, and Watanabe et al., "MPEG2 Interframe Predict System," The Television Society Technical Report, ICS'92-73, October 1992, discloses MPEG-2.
The above two articles relate to techniques for compressing image information by performing a motion compensation interframe prediction coding utilizing correlation between image frames, that is, correlation in time direction. That is, as shown in FIG. 4, a series of frame images which are continuous in time is classified into three picture types: (1) I picture for intra-frame coding, (2) P picture to be prediction-coded from an I or P picture in a past frame for motion compensation, and (3) B picture to be prediction-coded from I or P pictures in past and future frames for motion compensation.
P or B picture in a current frame is divided into macro blocks, each of m horizontal pixels.times.n vertical pixels. On the other hand, I or P picture of a reference frame is also block-divided and a motion vector is obtained between the current frame and the reference frame. A difference value, prediction error, is obtained every pixel between blocks of proximate images by utilizing the motion vector thus detected. When the difference is too large, the motion compensation is not performed and, instead, an original pixel value of the macro block in the current frame is selected.
The difference value or the pixel value is intra-frame coded by such as DCT (Discrete Cosine Transformation). The intra-frame coding is also performed for all blocks in the I picture and blocks called intrablocks to which the motion compensation prediction is not performed.
A compression of the amount of information to be done by combining the above-mentioned motion compensation prediction coding is also considered in the image signal coding by which compatibility is provided between the HD image and the SD image. Prior art of such compression method is disclosed in Yukizawa and Watanabe, "Frequency Domain Realization of Motion Compensated Interframe Prediction for Sub-band/Transform Coding," The Electronic Information and Communications Society, Technical Report, IE-91-82, 1991-11.
In this article, an HD image is divided into a plurality of sub-bands as shown in FIG. 3. Among the divided band signals, an LL band signal corresponds to the SD image. When a P picture is to be coded, a past I or P picture is also divided into sub-bands.
Then, a motion compensation prediction of the LL band signal of the P picture is performed from the LL band of the past I or P picture. A difference resulting from the prediction is coded by using a suitable method such as DCT.
Then, a motion compensation prediction of the LH band signal of the P picture is performed from the LH band of the past I or P picture. In this case, it is possible to use the motion vector of the block which is obtained for the LL band signal as the motion vector. This is because a certain block in the LH band signal and a block in the LL band signal, which corresponds to such certain block, are band-divided signals in the same position of the image and considered as having similar motion vectors.
Therefore, the motion compensation prediction of the block in the LH band signal is performed by using the motion vector obtained for the block in the LL band signal as it is or searching a relatively small range around the motion vector. The difference resulting from the prediction is similarly coded by using a suitable method such as DCT. The same processing of the LH band signal is performed for the HL and HH band signals.
For the I picture, intra-frame compression is performed for every sub-band divided band signal.
In the method in which the motion compensation is performed after the sub-band division, however, the coding efficiency is not as high as pointed out in the above-mentioned articles. This is because the respective band signals obtained by sub-band division are sub-sampled signals. That is, since the frequencies of the LH, HL, and HH band signals are high, the signal values at the sub-sampling points become different between the current frame and the reference frame and the correlation is not always high. Therefore, the difference between macro blocks of the current and the reference frames does not become small, and thus the code amount is not reduced as expected.